This invention relates to latexes and latex coatings.
Paper is often filled with mineral fillers such as clay, calcium carbonate and titanium dioxide. Such fillers are used to increase the opacity of the paper product. It is also advantageous to employ coatings and/or binders in the paper making process. For example, synthetic latexes or natural binders such as proteins or starch are employed as coatings alone or as components of pigmented coatings to increase the strength of the paper. The use of synthetic latexes as binders has become popular due to the desirable properties exhibited by said latexes.
Coatings are often applied to continuous web materials such as paper through the use of a high speed coating devices. For example, when a blade coater is employed, the properties of the coating which is applied to the paper can be varied by altering the blade thickness or the blade angle of the coater, the amount of pressure employed in forcing the coating material through the blade, or the rheology of the coating itself.
It is desirable that latexes which are applied using coating devices remain as discrete, stable, free moving particles in order to obtain trouble-free runability. However, when a latex containing coating formulation is subjected to high shear, such as, for example, in a blade coater, the formulation can exhibit a shear thinning or shear thickening behavior. Shear thickening can be reduced by decreasing the solids content in the coating formulation. Although a reduction in the amount of solids will improve the runability of the formulation, the quality of the resulting coating can be adversely affected by low coating weight or excess "diving in" of the coating into the paper substrate. It is desirable to have greater coating "hold-out" on the surface of the paper so as to achieve paper exhibiting improved printing quality.
Coating formulations which are increasingly high in solids facilitate the production of high quality coatings. Typically, high solids coatings are obtained by adding dry pigment to pigment slurries. However, a high solids latex reduces or eliminates the need for dry pigment addition. In addition, high solids coatings are desirable in increasing production rates and reducing energy costs. High solids versions of conventional latexes are limited to their use as paper coatings due to the resulting high coating formulation viscosity at high shear rates. This leads to poor blade coater runability. Typically, poor runability is characterized by scratching or streaking of the coating, or lack of coat weight control (i.e., very high weight coatings and/or uneven coatings).
Improved high solids formulations for coating paper are disclosed in U.S. Pat. No. 4,474,860. The disclosed formulations comprise a high solids synthetic bimodal latex comprising two separate and distinct particle size distributions of styrene/butadiene type latexes. Such formulations are disclosed as exhibiting good runability during application using a device such as a blade coater. However, it would be desirable to provide a formulation for coating paper which, in addition to the previously described desirable properties, exhibits high sheet gloss when applied to paper, exhibits a high ink gloss after inking of paper is performed, exhibits a high porosity and ink receptivity when applied to paper, and exhibits good binding strength.
In view of the deficiencies in the prior art, and in view of the desirability of providing improved paper coating formulations, it would be highly desirable to provide a means of preparing a paper coated with a formulation comprising a high solids synthetic latex which exhibits good runability during application with a device such as a blade coater.